Recently, problems relating to the rapid increase in the demand for power, depletion of fossil fuels, and environmental pollution have become more and more serious. Therefore, interest in a fuel cell that can resolve the problems of this environmental pollution along with the supply and demand of energy has been increasing more and more.
The general efficiency of an internal combustion engine is 10˜20%, however, the fuel cell can increase efficiency over 75% by using the heat generated in the fuel cell. Also, the quantity of nitrogen compound or sulfur compound contained in exhaust gas is very low, and almost no noise or vibration results.
Also, the fuel cell can be produced in various capacities, equipped within a power demand area, and therefore decrease equipment relating to the transmission and transformation of electricity.
The following table 1 represents various kinds of fuel cells.
TABLE 1PhosphoricPolymerAcid FuelMolten CarbonateSolid OxideElectrolyte FuelAlkaliCellsFuel CellsFuel CellsCellsFuel CellsAbbreviationPAFCMCFCSOFCPEFCAFCGasHydrogenHydrogen,Hydrogen,HydrogenHydrogencarbon monoxidecarbon monoxideElectrolytePhosphoric acidLithium carbonate,StabilizedCationic exchangePotassiumPotassium carbonatezirconiamembranehydroxideIonHydrogen IonCarbonic acid ionOxygen ionHydrogen ionHydroxyl ionOperatingAbout 200° C.About 650° C.About 1000° C.normalnormaltemperaturetemperature~temperature~about 100° C.about 100° C.Efficiency40~50%45~60%50~60%40~60%45~60%MaterialsCity gas LPG etcCity gas, LPG, coalCity gas, LPG etcCity gas, LPG etcHydrogenetc
Referring to table 1, various kinds of fuel cells are as follows. That is, they include Molten Carbonate Fuel Cells(MCFC) which operate in high temperature over 600° C., Phosphoric Acid Fuel Cells(PAFC) which operate in comparatively low temperature under 200° C., Solid Oxide Fuel Cells(SOFC) and Polymer Electrolyte Fuel Cells(PEFC) and so on. Also, Direct Methanol Fuel Cells (DMFC) use methanol as the fuel.
Over all, the Polymer Electrolyte Fuel Cells (PEFC) use a polymer electrolyte membrane having the characteristic of exchanging a hydrogen ion as the electrolyte. Also, the Polymer Electrolyte Fuel Cells (PEFC) are called various names like Solid Polymer Electrolyte Fuel Cells (SPEFC) or Proton Exchange Membrane Fuel Cells (PEMFC), however, they will be called Polymer Electrolyte Fuel Cells (PEFC) in the following.
The Polymer Electrolyte Fuel Cells(PEFC) have a low operating temperature of about 80° C., high efficiency, good current and output power density, short warming time, and rapid response to change of load in comparison with the other fuel cells.
Especially, the Polymer Electrolyte Fuel Cells (PEFC) are unnecessary to control an electrolyte and less sensitive to pressure change of reaction gas because a polymer electrolyte membrane is used as an electrolyte. Also, its design is simple, manufacture is easy, and output range is various. Therefore, the Polymer Electrolyte Fuel Cells (PEFC) are applied to various field like a power source for a pollution free vehicle, on-the-spot generation of electric power, a mobile power source, a military power source and so on.
The basic principle of the fuel cell is to make electricity and water from oxygen and hydrogen. Namely, chemical energy is transformed to electric energy directly in steady combustion of fuel and hydrogen supplied from the outside.
The reaction formula in each electrode is as follows.
The reaction formula in anode is as Reaction Formula 1.H2(g)2 2H+2e−  [Reaction Formula 1]
The reaction formula in cathode is as Reaction Formula 2.
[Reaction Formula 2]
            1      2        ⁢                  O        2            ⁡              (        g        )              +      2    ⁢          H      +        +      2    ⁢          e      -        ⁢    2    ⁢                  ⁢          H      2        ⁢    O  
The total reaction formula is as Reaction Formula 3.
[Reaction Formula 3]
            H      2        ⁡          (      g      )        +            1      2        ⁢                  O        2            ⁡              (        g        )              ⁢    2    ⁢                  ⁢          H      2        ⁢    O  
FIG. 1 is a schematic cross section of a unit cell that consists of conventional fuel cells.
The fuel cell stack includes a plurality of membrane electrode assemblies piled. And the fuel cell comprising only one membrane electrode assembly is called a unit cell. The real fuel cell stack has the structure of unit cells piled.
The voltage of one membrane electrode assembly is about 0.7V, and the output voltage of the stack increases in proportion to the number of membrane electrode assemblies. The current is in proportion to the area of the membrane electrode assembly, therefore the wider the area is, then the more the current is generated. Generally, the current density of membrane electrode assembly is about 200–500 mA/cm2.
Referring to FIG. 1, in the anode 100 of unit cell the electron is generated as the result of oxidation reaction occurring in supplying and exhausting hydrogen. And the water is generated as the result of a reduction reaction occurring in supplying and exhausting oxygen.
The oxidation and reduction occurs in the catalyst layer 130, which is formed between two porous carbon electrodes and the polymer electrolyte membrane The polymer electrolyte membrane 120 delivers the hydrogen ionized in the anode 100 to a cathode 110.
The membrane electrode assembly is manufactured by hot-pressing of the polymer electrolyte 120 and the catalyst layer 130. The anode 100 and the cathode 110 of PEFC are comprised1 of the catalyst layer 130 and the supporting bodies 100.
Separators (also, they are called bipolar plates) 160, 165 on which a flow field 140 is formed, can supply hydrogen and oxygen and exhaust the water and is located outside of the membrane electrode assembly.
The separator is a conductive plate including the flow field on one side, and the hydrogen and oxygen can move through the flow field. The separator conducts the electron generated in anode to cathode, supports the membrane electrode assembly, supplies two electrodes with fuel and oxygen, and removes water generated while operating the fuel cell.
The unit cell includes the membrane electrode assembly, the separator, and a gasket 170,180 preventing the gas or the liquid from flowing out. Also, the unit cell includes a copper plate 190,195 which can fix the unit cell in the end portion of the unit cell.
The fuel cell stack is manufactured by piling the unit cells. The hydrogen gas channel is formed on one side of the separator, and the oxygen gas channel is formed on the other side of the separator.
The problem with the past technique will be explained in connection with improving the performance of membrane electrode assembly.
In order to improve performance of the membrane electrode assembly, it is necessary to promote an oxidation-reduction reaction through the efficient use of a catalyst, decrease contact resistance between the polymer electrode assembly and the electrodes, increase the area of the three-phase boundary of gas-electrolyte-catalyst, and lessen the thickness of the polymer electrode assembly.
Therefore, research and development have been undertaken to improve the polymer electrolyte membrane, promote the rate of catalyst utilization, improve the catalyst coating process, increase the area of the three-phase boundary of gas-electrolyte-catalyst, and decrease contact resistance between the polymer electrode assembly and the electrodes.
However, improving the performance of the membrane electrode assembly through the above-mentioned method may be difficult.
In the following, conventional representative methods for improving performance of the membrane electrode assembly and the problems corresponding to the methods will be explained.
The first method is to improve the polymer electrolyte membrane. The polymer electrolyte membrane must have high conductivity of hydrogen ion and almost no conductivity of electron. Also the polymer electrolyte membrane must have less movement of reaction gas or water than that of an ion but good mechanical and chemical stableness.
The Nafion membrane (Dupont Co.) used in PEFC is perfluorinated sulfonic acid series. The hydrogen must be hydrated to be conducted in the Nafion membrane and passes through in the Nafion in the form of H3O+.
Therefore, if the polymer electrolyte membrane becomes dry, the conductivity of hydrogen ion becomes low, and the polymer electrolyte membrane will contract to increase contact resistance between the polymer electrolyte membrane and the electrodes. That is to say, the thinner the polymer electrolyte membrane is, the less the ohmic resistance of hydrogen ion is, and consequently the current density increases.
However, it is difficult to lower the thickness of polymer electrolyte membrane because of mechanical solidity. The thickness of Gore-Select membrane (Gore Associate Co.) having a good mechanical property of matter is about 20˜30 μm.
Accordingly, a new polymer electrolyte membrane needs to be developed which does not require humidification, can be operated in high temperature over 200° C., and has good mechanical and chemical duration. However, there is no the polymer electrolyte membrane that complies with the qualifications mentioned above.
The second method is to improve the efficiency of a catalyst. The oxidation and reduction of hydrogen gas and oxygen gas occurs in catalyst layer. Therefore, the efficiency of a catalyst is very important.
However, it is necessary to improve the efficiency of a catalyst by improving a dispersion degree of the catalyst because the catalyst is a precious metal.
The representative method for improving the efficiency of a catalyst is to use supported Pt/C whereby platinum is coated on a surface on a minute carbon particle of 2˜5 nm size (Vulcan XC-72, Cabot Co.).
However, the above-mentioned method that can increase the effective surface area of a platinum particle by using a small quantity of a catalyst can't give prominent performance improvement.
A uniform and thin catalyst layer can decrease the contact resistance. Namely, lessening the thickness of the polymer electrolyte membrane can decrease the voltage drop in PEFC.
However, the thickness of the polymer electrolyte membrane must be sufficiently thick to support the membrane electrode assembly. Therefore, the reproducibility and stableness of a method for producing the membrane electrode assembly restricts the thickness of the polymer electrolyte membrane to be hundreds of micrometer.
The third method is to improve the degree of homogeneousness. Namely, during hot-pressing the degree of homogeneousness can be increased by controlling temperature, pressure, time, and humidity. However, a high possibility exists that the degree of homogeneousness becomes low and contact resistance becomes high in a preponderance of pressure because the polymer electrolyte and the electrodes are contacted on flat side.
As mentioned above, the method for improving the performance of the membrane electrode assembly is concentrated on the improvement of material comprising the membrane electrode assembly or the activation of reaction centering around the catalyst.
On the contrary, no method exists for increasing a reaction effective method or decreasing the contact resistance through improving the structure of the membrane electrode assembly.